This invention relates to an optical switching module for switching lightwave signals in a telephone switching system or the like, more particularly to an optical switching module configured for reduced size.
Increased use of fiber-optic communications has given rise to the need to switch lightwave signals without opto-electric and electro-optic conversion. For example, in a switching system comprising a plurality of subscriber line concentrator modules which exchange lightwave signals with a plurality of circuit switching modules via an inter-module connector, the connector needs to be able to switch the connections among modules so that a faulty circuit-switching module can be replaced by a standby module. The inter-module connector, accordingly, preferably includes one or more optical switching modules.
One type of optical switching module comprises a matrix of thermo-optic switching elements interconnected by waveguides formed on a silica substrate. Switching is accomplished by the use of thin-film heaters to vary the temperature of the switching elements. The module also includes electronic circuits for feeding switching current to the heaters, and a heat sink to dissipate heat caused by the switching operations.
Adjustment of the switching current is conventionally carried out manually, by mechanically adjusting variable resistors such as trimmer potentiometers provided in the electronic circuits. A disadvantage of this arrangement is that it consumes both space and time. Mechanically adjustable variable resistors are comparatively large devices that take up space in the switching module, and adjusting them manually is a slow, inefficient process.
Attachment of optical fibers causes a further problem. Conventionally, fiber pigtails are butt-coupled to waveguides in the switch matrix by an adhesive, then secured to the heat sink at a certain distance from the butt joints to protect the butt joints from mechanical stress. A disadvantage of this arrangement is that to provide a platform for securing the fiber pigtails, the heat sink must be physically larger than the switch matrix; that is, it must be larger than necessary simply to dissipate the heat produced by switching.
The heat sink comprises fins projecting from a flat base. The height of the fins is proportional to the overall size of the heat sink, so increasing the length of the heat sink to provide supports for the fiber pigtails has the further disadvantage of increasing the height of the module assembly.